This invention relates to a process for making an extruded thermoplastic foam structure with a first blowing agent and a second blowing agent comprising water.
Because of environmental concerns, it is desirable to employ inorganic blowing agents such as carbon dioxide, nitrogen, and water in making thermoplastic polymer foam structures. Water has utility because of its relatively high blowing power and cooling capability.
A problem with using water as a blowing agent is that it may increase corrosion in process equipment. The problem is particularly acute when a brominated aliphatic fire retardant is employed. Water can combine with hydrogen bromide (HBr) or other decomposition products of brominated aliphatic fire retardants resulting in polymer melts and gels of polymer melts and blowing agents with relatively high acid content. The problem is particularly acute at process temperatures of about 160.degree. C. or more, the range at which potentially troublesome levels of decomposition typically occur in commonly-employed brominated fire retardants.
One means of addressing the acid problem is to employ process surfaces and equipment comprised of acid-resistant alloys or materials. Employing such equipment is undesirable due to their considerable cost, which is typically much higher than non acid-resistant alloys and materials.
Another means of addressing the acid problem is to employ acid scavengers, i.e. compounds having a basic moiety, to neutralize the acid content of the polymer melt. Acid scavengers are commonly employed in making foams, but there are practical limits as to their effectiveness and how much may be employed. High levels of acid scavengers may interfere with the fire-retarding performance of the flame retardant as well as physical properties of the end product foam structure.
It would be desirable to minimize corrosion of process surfaces and equipment when making foams with water as a component of the blowing agent. It would further be desirable to accomplish this while minimizing equipment costs or adverse impact to the physical and flame-retardant properties of the end product foam structure.